There is one problem universal to bibliophiles the world over: just where are you going to store all those books? It's a never-ending conundrum; once you've solved the problem of storing the books you have now, where on earth are you going to store all the books you're likely to have in another ten years time?
Where some people build conservatories or garages or sheds, the bookworm's dream is to build a library. It's not actually a crazy idea, either; like most home extensions, the extra space will add value to your property, and the bliss of having a reading space all of your own is second to none. If you want to get the most out of your library, though, you're going to need to plan for it properly--including leaving yourself some room for your collection to expand.
Make sure your books will be adequately protected from damp and sun damage.
Many home extensions are designed for use as conservatories or sun rooms, and they sometimes aren't held to the same high standards as existing buildings. If you're planning to use your extension for book storage, however, it's absolutely vital that it is as dry and protected as possible--so it's important to hire contractors who are used to working on houses as well as extensions and that you make sure they know the extension's purpose before they begin work.
Plan ahead to make the most use of every inch of space.
Think laterally to ensure you're not missing potential book storage space. Ideally you want to have every shelf in place before you start moving your books to their new home, to make it easier to get everything into a sensible order right from the start. Don't forget to factor in the space in the middle of the room--bookshops and libraries use aisles because it's the most efficient way, after all.
Choose your sorting system in advance, to save pain later.
Sorting fiction is easy--alphabetical order by surname of the first author listed, and then by publication date. Non-fiction presents a few extra challenges, though; the Dewey decimal system really isn't designed for personal use, and isn't ideal for collections where there are only a few distinct subjects but they're extremely well-covered. Your best bet is to come up with your own system, based on your own knowledge of your collection and how often you're likely to want to find specific volumes--but it's important to think about this in advance.